Dabate deposit is a typical porphyry Cu-Mo deposit in the Western Tianshan, Xinjiang, China. Both the Cu and Mo mineralization are mainly associated with intrusion of rhyolite porphyry. Distinguishing from typical porphyry deposits, the Cu mineralization and Mo mineralization are separated from each other on both the temporal and spatial scales. The Cu orebodies are mainly hosted in the contact zone between the rhyolite porphyry and Tuosiku'ertawu Formation and predominantly occur as fluorite-arsenopyrite-Cu-sulfides ± K-feldspar ± quartz veins (Cu-1 stage), whereas the Mo orebodies are mostly hosted in the inner part of the rhyolite porphyry and are typically presented as Mo-2 stage quartz-molybdenite veins (external Cu and internal Mo). Total homogenization temperatures (Th, L-V) for boiling fluid inclusion assemblage of the Mo-2 stage occurred at 302 to 358 °C, with corresponding salinities of 1.4 to 38.1 wt% NaCleqv. H-O isotope data indicate that the Mo mineralization fluids were mainly derived from magmatic water. The non-boiling fluids of the Cu-1 stage originated from magmatic fluids and meteoric water, which were characterized by high temperature (323 to 379 °C) and low salinity (2.6 to 5.9 wt% NaCleqv) as well as Cu, As, and volatile enrichment. These features are different from the initial magmatic fluids of this deposit, indicating that the Cu mineralization may occur before the Mo mineralization (early Cu and late Mo) and the magmatic fluid components of the Cu-1 stage should be derived from the condensation of the low-salinity vapor generated by boiling of the hydrothermal fluids (351 to 405 °C; 1.8 to 40.1 wt% NaCleqv) for the earliest Mo-1 stage barren quartz veins. The positive δ34S values (5.5 to 9.8‰) of sulfides indicate that the sulfur in the ore-forming fluids was derived from magma and country rocks. The subtle change in fluid conditions from the high temperature (peak range 340 to 360 °C), alkaline, and slightly more oxidized of the Cu-1 stage to high temperature (peak range = 320 to 340 °C), more reduced, and acidic of the Mo-2 stage gave rise to the temporal separation of Cu and Mo mineralization. The Cu, As, and volatiles (e.g., CO2, HF), preferentially partitioned into the vapor phase, were highly enriched in Cu ore-forming fluids, whereas Mo mineralization fluids contained the majority of Mo with strong preference for the brine phase. Therefore, the spatial separation of the Cu and Mo mineralization could be attributed to selectively fractionating between the vapor phase and hypersaline liquid of the Cu and Mo metals during the process of fluid boiling.
The Jiamante Au deposit is one of the important deposits in the Axi ore cluster of the Western Tianshan, Xinjiang. The orebodies occurring mainly as quartz‐sulphide veins are hosted in Late Devonian volcanoclastic rocks and granitic porphyries, and structurally controlled by a group of NNW‐ and NNE‐ trending extensional faults and volcanic breccias. Primary metallic minerals are chalcopyrite, galena, pyrite, sphalerite, and native gold, with minor amounts of bismuthinite and bismuth. Nonmetallic minerals are dominated by quartz, sericite, chlorite, and calcite. Medium to low temperature hydrothermal alterations are well developed and characterized by silicification, beresitization, phyllic alteration, and chloritization. Two hydrothermal stages were identified: (a) the main‐ore quartz‐sulphide stage; and (b) the post‐ore quartz‐calcite stage. Microthermometric measurements indicate that the primary biphase liquid‐rich aqueous fluid inclusions within quartz of the main‐ore stage homogenize at temperatures ( T h ) of 176 to 251°C and have salinities of 2.1 to 7.4 wt% NaCl eqv . A lower range of homogenization temperatures (169 to 212°C) and salinities (1.6 to 5.2 wt% NaCl eqv ) were obtained in calcite of the post‐ore stage. The fluid characteristics of moderate to low temperatures and salinities imply that the ore‐forming process occurred in an epithermal environment. In addition, there is a positive relationship between T h and salinities consistent with the fluid mixing trend. Together with H–O isotopic data ( δ D H2O = −118 to −97‰ and δ 18 O H2O = −5.5 to −1.6‰) and fluid characteristics, it is suggested that the ore‐forming fluids were a mixture of magmatic and meteoric waters. The δ 13 C values for CO 2 in the hydrothermal fluids range from −3.2 to −2.7‰, and the sulphides have δ 34 S values of −5.2 to 2.9‰ with relatively uniform Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.228 to 18.287, 207 Pb/ 204 Pb = 15.574 to 15.617, and 208 Pb/ 204 Pb = 38.151 to 38.251), indicating that the ore‐forming materials were magmatic sources that were derived directly from magmatic fluids and/or leached from volcanic‐subvolcanic rocks by hydrothermal fluids. Combined with the evidence from the mineralization and alteration, fluid inclusions, and C–H–O–S–Pb isotopes suggest that the Jiamante deposit is an intermediate‐sulphidation epithermal Au deposit. A combination of fluid mixing and fluid–rock interactions is inferred as the important factors causing ore deposition. According to the close spatial and temporal relationship between intermediate‐epithermal deposits and porphyry deposits, it is further suggested that porphyry‐type mineralization should have a good potential at the deep sites of the Jiamante deposit and its vicinity.
Abstract Scanning electron microscope, cathodoluminescence (CL) imaging and laser ablation–inductively coupled plasma–mass spectrometry analyses were conducted on coexisting pairs of quartz and K-feldspar from 14 samples of various types of felsic igneous rocks. Difference of the concentration of trace elements in quartz and K-feldspar among plutonic, pegmatitic and volcanic rocks is closely related to the rock-forming process and P–T condition. In general, a decreasing Ti concentration and increasing Al, Li, and Ge concentrations from plutonic to pegmatitic quartz suggest a higher degree of magmatic fractionation and lower crystallization temperature. Sensitive elements to magma differentiation in K-feldspar such as Ba, Sr, and LREE show a decreasing trend from various granitic rocks to pegmatite, while Rb, Cs, Li, Ge, and P exhibit increasing trends. The K-feldspar from various types of felsic igneous rocks typically shows similar CL textures but relatively higher luminescence intensity compared with coexisting quartz. Both quartz and K-feldspar phenocrysts in the volcanic rhyolite have a bright rim and a dark core in CL images, corresponding to bimodal Ti concentrations. Among all samples, Ti concentrations in both quartz and coexisting K-feldspar positively correlate with their CL intensities, suggesting the activation of Ti-impurity in these minerals leads to increased CL intensity. Meanwhile, there is a good positive correlation between Ti concentrations in quartz and those in K-feldspar with an R2 value of 0.86. It is considered that Ti concentrations in the both minerals are mainly temperature dependent at relatively constant pressure on basis of a fair aTiO2 restriction. Here, for the first time, we calculated a titanium-in-K-feldspar (TitaniKfs) thermometer in the form of log (XTi, kfs/aTiO2) = −(3430 ± 268)/T(K) + (5.081 ± 0.298) for natural felsic magma systems. The Ti contents of K-feldspar (in ppm by weight) increase exponentially with reciprocal T at temperatures ranging from 500°C to 800°C, at 200–300 MPa. An application of this thermometer to granitic rocks from Xinjiang and Inner Mongolia in China fits well with other geothermometers. In addition, the present TitaniKfs thermometer is expected to be particularly useful in determining the temperature condition of K-feldspar-bearing extraterrestrial materials such as lunar rocks.
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